In the field of automotive manufacturing, structural components that historically have been fabricated of steel, such as engine cradles, are increasingly being replaced with aluminum alloy castings. Such castings are typically large, convoluted, and relatively thin, and are required to meet the high quality standards of automotive manufacturing. In order to meet these requirements, vacuum-assisted die-casting is typically used to produce such castings.
Vacuum-assisted die-casting machines comprise a piston, sometimes referred to as a “plunger”, that is advanced through a piston bore, sometimes referred to as a “shot sleeve”, to push a volume of liquid metal into a mold cavity. Vacuum is applied to the piston bore to assist the flow of the liquid metal therethrough. A replaceable wear ring is fitted onto the piston, and makes continuous contact with the inside of the piston bore along the full stroke of the piston for providing a seal for both the vacuum and liquid metal. The wear ring sits freely on a circumferential rib rearward of the front face of the piston head, and is split to allow it to be installed onto the piston head prior to use, and to be removed from the piston head after use.
For example, FIG. 1 shows a portion of a prior art vacuum-assisted die-casting apparatus, which is generally indicated by reference numeral 20. Vacuum-assisted die-casting apparatus 20 comprises a piston that is moveable within a piston bore 22 defined within a shot sleeve 24 for pushing a volume of liquid metal (not shown) into a die-casting mold cavity (not shown) to form a casting. In the example shown, the piston is positioned at its starting position of the stroke, which is rearward of a port 26 through which the volume of liquid metal is introduced into the piston bore 22.
The piston comprises a piston head 30 mounted on a forward end of a piston stem (not shown). The piston head 30 has a front face 32 that is configured to contact the volume of liquid metal introduced into the piston bore 22 via port 26. The piston head 30 has a wear ring 34 disposed on an outer surface thereof.
In operation, at the beginning of a stroke cycle, the piston is positioned at its starting position in the piston bore 22, and a volume of liquid metal is introduced into the piston bore 22 forward of the piston head 30 via port 26. The piston is then moved forward through the piston bore 22 to push the volume of liquid metal into the mold cavity for forming a metal casting, and is then moved rearward to its starting position to complete the stroke cycle. During this movement, the wear ring 34 disposed on the piston head 30 continuously contacts the surface 38 of the piston bore 22, and provides a liquid metal seal for preventing liquid metal from passing between the piston head 30 and the surface 38 of the piston bore 22. The wear ring 34 also provides a vacuum seal for maintaining vacuum (that is, a low pressure) within the forward volume of the piston bore 22. The cycle is repeated, as desired, to produce multiple metal castings.
Other die-casting pistons have been described. For example, U.S. Pat. No. 5,048,592 to Mueller discloses a plunger for forcing molten aluminum or brass out of a casting cylinder of a die-casting machine. The plunger includes a cap that is screwed via an internal thread onto an external thread of a supporting body and is made of a material, in particular a copper alloy, which has a greater coefficient of thermal expansion than the material of the cylinder, in particular steel, and the material of the supporting body, in particular steel. In one embodiment, the cap has on its outer cover face a cylindrical extension with an outer annular web, which engages into a corresponding inner annular groove of a sealing ring. The sealing ring is split radially in a step shape.
U.S. Pat. No. 7,900,552 to Schivalocchi et al. discloses a piston for a cold chamber die-casting machine comprising a body and at least one sealing band mounted around the body. The body and the band are provided with coupling means for obtaining both an angular locking and an axial locking of the band to the piston body.
U.S. Pat. No. 8,136,574 to Müller et al. discloses a multi-piece piston for fixing to a high pressure side end of a piston rod running axially in a casting cylinder of a cold chamber casting machine. The piston comprises a piston crown forming a piston front face on the high pressure side and a piston body in the form of a bush connected to the piston crown on the low pressure side. Complementary bayonet locking means are provided for axial fixing of the piston to the end of the piston rod, on the piston crown and the end.
Improvements are generally desired. It is an object at least to provide a novel die-casting piston, and a die-casting apparatus incorporating the same.